Generally, a nitrogen oxide (NOx) included in exhaust gas is a cause of acid rain, harms eyes and respiratory organs, and withers plants. The NOx is regulated as a major air pollutant, and much research has been carried out in order to reduce an amount of the NOx in exhaust gases.
An exhaust gas recirculation (EGR) system is provided in a vehicle for reducing noxious exhaust gas. Generally, the NOx is increased in a case where an air-fuel ratio of an air-fuel mixture is high, which may be necessary for sufficient combustion. Thus, the exhaust gas recirculation system mixes some (e.g., 5-20%) of exhaust gas from an engine with the air-fuel mixture, thereby reducing an amount of oxygen in the air-fuel mixture and retarding combustion, thus suppressing generation of the NOx.
A typical exhaust gas recirculation system recirculates some exhaust gas that is exhausted from a cylinder of an engine through an exhaust manifold to flow back to the cylinder of the engine, through a recirculation line. A recirculation valve is mounted in the recirculation line to control an EGR rate.
However, in a conventional recirculation valve, when a foreign material is accumulated inside the recirculation valve due to an abnormal cause, some recirculation gas may leak into a combustion chamber of the engine through the recirculation valve, even though the recirculation valve is closed.
As such, combustion stability of the engine may be degraded when the recirculation gas leaks into the combustion chamber in the case that the recirculation valve is to be closed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.